1. Field of the Invention
The present invention is related to a self-test circuit for a multi-point level indicator.
2. Description of the Prior Art
In many industries, it is desirous to monitor the level of the contents of various bins or hoppers. For example, if a hopper is used for filling other containers, an operator must know when the hopper is full so that he can empty the hopper to prevent any overflow. Similarly, an operator must know when the hopper is emptied so that he can have it refilled.
Previously, the level of the fill material in a hopper was monitored by using a single point level gauge. However, a single point gauge can only monitor the level of a single hopper. Consequently, in a factory which contains a plurality of hoppers whose levels must be monitored, a separate single point level gauge must be used for each hopper. Some prior systems have incorporated a plurality of such single point gauges into one apparatus, such that the system has the capability of monitoring more than one hopper. However, these systems merely duplicate the complete gauge for each hopper.
Prior art level indicator systems have typically utilized a radiation beam for monitoring the level of the contents in a hopper. For example, a radiation source is positioned to direct a radiation beam through a hopper at a point where the level of the contents is to be detected. A radiation sensor positioned on the opposite side of the hopper to monitor the amount of radiation which passes through the hopper. When the beam must pass through the hopper contents, the amount of radiation received by the sensor is relatively low, since much of the radiation will be absorbed. On the other hand, if the beam passes through the hopper without passing through the contents, the amount of radiation received by the sensor is relatively high. However, one problem associated with these types of radiation sensors is that they are typically prone to a relatively high failure rate. Thus, an operator must constantly check the sensor to ensure that it is operating properly.
Another problem associated with both radiation sensors and other types of sensors occurs when there is a build-up of fill material on the side walls of the hopper. In these instances, a level gauge monitoring a hopper which has just been emptied may indicate that hopper is full, since the gauge will detect the fill material on the side walls of the hopper. Thus, an operator must periodically inspect the hoppers to make sure the side walls of the hopper are relatively free of fill material.